The state of the art relevant to this invention falls into two general categories: 1) covalent modification of H-terminated Si surfaces, and 2) the μ-contact printing techniques. These two topics are summarized below.
A. Covalent Modification of H-Terminated Si Surfaces.
Many methods have been developed for attaching organic molecules to H-terminated Si surfaces via Si—C bond formation (Buriak, 2002, Chem. Rev. 102, 1271). These include:                1. Hydrosilylation involving a radical initiator.        2. Thermally induced hydrosilylation.        3. Photochemical hydrosilylation.        4. Hydrosilylation mediated by metal complexes.        5. Reactions of alkyl/aryl carbanions.        6. Electrochemical diazonium reactions.        7. [2+2] Reactions of alkynes and alkenes with reconstructed Si(100).        8. Diels-Alder ([4+2]) reactions of dienes with reconstructed Si(100).        9. Halogenation followed by alkylation.        10. Anodic grafting.        
In addition, strategies have been developed for attaching organic molecules to H-terminated Si surfaces via Si—O—C or Si—N—C bond formation. These include:                1. Halogenation followed by reaction with alcohol or amine (Rogozhina, E. et al. (2001) Appl. Phys. Lett. 78, 3711; Zhu, et al. (2001) Langmuir 17, 7798).        2. Reaction with alcohol in the presence of halogen and one-electron oxidant (Haber, J. A.; Lewis, N. S. (2002) J. Phys. Chem. B 106, 3639).B. Patterning, in Particular Microcontact Printing (μCP).        
Presently, there are six techniques normally referred to as soft lithography: microtransfer molding, replica molding, micromolding in capillaries, solvent-assisted microcontact molding, near field phase shifted lithography and microcontact printing (also often abbreviated as μCP). The latter is the most commonly used and investigated soft lithography technique. For μCP, a liquid polymer precursor (usually polydimethylsiloxane, PDMS) is poured over a master that has been produced by photo- or electron beam lithography. After curing, the PDMS stamp with the desired pattern is peeled off the master (Xia and Whitesides, 1998, Annu. Rev. Mater. Sci. 28, 153). There are two possible methods of inking the stamp (Michel et al., 2001, IBM J. Res. & Dev. 5, 697): immersion inking and contact inking. For immersion inking, the stamp is inked with a solution and subsequently dried. For contact inking, the stamp is simply pressed on an inkpad, which is usually a block of PDMS that was previously treated with the ink solution. Subsequent pressing of the inked stamp on a substrate transfers the molecules contained in the ink from the stamp to the substrate.
The elasticity of the stamp is one of the parameters that determine the resolution limits of the soft-lithography (Michel et al., 2001, see above). Commercial PDMS, with a Young's modulus of 3 MPa, is too soft to define structures smaller than 500 nm. The hardness of the polymer can be changed by varying the ratio of pre-polymer to cross-linker and therefore the molecular mass MC between the cross-links or by mixing different types of polymer precursors (Schmid and Michel, 2000, Macromolecules 33, 3042). By using bimodal polymers having two populations of chain lengths, stamp materials with a Young's modulus of 9.7 MPa and sufficient toughness for large area printing can be made. Features as small as 80 nm can be achieved with these hard polymers. Currently the smallest feature size realized by μCP is <50 nm over 3×3 mm2 using “hard” PDMS stamps and dendrimers as ink molecules (Li et al., 2003, Langmuir 19, 1963).
Another important factor determining the resolution limit of soft lithography to which much attention has been paid is diffusion of the ink molecules on the substrate surface. Several diffusion pathways of the ink molecules were considered (Delamarche et al., 1998, J. Phys. Chem. 102, 3324): simple spreading across the contact surface and diffusion from non-contact zones of the stamp to the surface, either along the stamp material or via the vapor phase. The molecular weight of the ink influences the vapor pressure and the diffusion path length. Higher molecular mass means lower diffusion and lower vapor pressure. In addition, the influence of ink concentration and printing time of contact has been investigated (Delamarche et al., 1998, see above).
Most μCP techniques for printing organic molecules onto silicon require a layer of silicon oxide in between, which introduces an electrically insulating barrier between the printed layer and the bulk silicon phase. On the other hand, most methods for attaching organic molecules directly to bulk silicon surfaces are not suitable for μCP. An exception is the process developed by Jun and Zhu (2002, Langmuir 18, 3415), but it requires the use of chlorine gas and the printing, which is performed in a glove-bag at 70° C., requires 30 minutes or longer.
Accordingly, it was an object of the present invention to provide for a method that allows for quick printing and/or patterning of molecules, preferably organic molecules onto silicon surfaces.